Determine actual base URI from the model

Because we're managing more than one version of the model, it would be better to set the base URI dynamically instead of using a predefined constant.

There can be at least two ways of doing this:

1. Get it from command line/environment variable
2. Get it from model Markdown source file (model Markdown source file will be the single source of truth)

Option (2) is possible because SPDX 3 model Markdown source contains the profile namespace IRIs (in profile metadata section).

We can do it like this:

iri = "https://spdx.org/rdf/3.0/terms/Core/"
if iri.endswith("/Core/"):
    base_uri = iri[: -len("Core/")]
elif iri.endswith("/Core"):
    base_uri = iri[: -len("Core")]

# base_iri = "https://spdx.org/rdf/3.0/terms/"

Start by locating the predefined base URI and the code that processes the SPDX 3 model Markdown profile metadata. Compare deriving the URI from command-line or environment input with deriving it from the Markdown source, then confirm the parser uses the selected value consistently. Done means the base URI no longer depends on a fixed version-specific constant.